patternjavaCritical

Removing objects from a collection in a loop without causing ConcurrentModificationException

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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fromobjectscausingconcurrentmodificationexceptionwithoutremovingloopcollection

Problem

We all know you can't do the following because of ConcurrentModificationException:

for (Object i : list) {
    if (condition(i)) {
        list.remove(i);
    }
}

But this apparently works sometimes, but not always. Here's some specific code:

public static void main(String[] args) {
    Collection list = new ArrayList<>();

    for (int i = 0; i 

This, of course, results in:

Exception in thread "main" java.util.ConcurrentModificationException


Even though multiple threads aren't doing it. Anyway.

What's the best solution to this problem? How can I remove an item from the collection in a loop without throwing this exception?

I'm also using an arbitrary

Collection here, not necessarily an ArrayList, so you can't rely on get`.

Solution

Iterator.remove() is safe, you can use it like this:

List list = new ArrayList<>();

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Note that Iterator.remove() is the only safe way to modify a collection during iteration; the behavior is unspecified if the underlying collection is modified in any other way while the iteration is in progress.

Source: docs.oracle > The Collection Interface

And similarly, if you have a ListIterator and want to add items, you can use ListIterator#add, for the same reason you can use Iterator#remove — it's designed to allow it.

In your case you tried to remove from a list, but the same restriction applies if trying to put into a Map while iterating its content.

Code Snippets

List<String> list = new ArrayList<>();

// This is a clever way to create the iterator and call iterator.hasNext() like
// you would do in a while-loop. It would be the same as doing:
//     Iterator<String> iterator = list.iterator();
//     while (iterator.hasNext()) {
for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
    String string = iterator.next();
    if (string.isEmpty()) {
        // Remove the current element from the iterator and the list.
        iterator.remove();
    }
}

Context

Stack Overflow Q#223918, score: 1677

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